This invention relates to a superconducting magnet apparatus mounted on a magnetically levitated train and, more particularly, to a superconducting magnet apparatus mounted on a magnetically levitated train in a magnetic guidance system in which ground coils for guidance arranged on both sides of a guideway are connected by null-flux lines, wherein the superconducting magnet apparatus is arranged to oppose the guidance ground coils.
As illustrated in FIG. 1, superconducting magnets 10 are disposed in left-right symmetry on both sides of a truck 12 located below the floor of a vehicle 11, and each superconducting magnet 10 internally incorporates a plurality of superconducting coils 1. Ground coils 13 for guidance are arranged on side walls 14 of a guideway.
In the case of a specific example of a superconducting magnet used in the Magnetically Levitated Vehicle MLU 002 of the Miyazaki Test Track, Kyushu, Japan, developed by the RTRI (Railway Technical Research Institute) and illustrated on pages 18 and 19 of "Approach to the 21st Century at 500 km/h" published by the Transportation News, one superconducting magnet is disposed on each of the two sides of the truck, and each superconducting magnet internally accommodates superconducting coils. FIG. 2 shows the circuitry of a conventional superconducting magnet apparatus for a magnetically levitated train in a case where three pairs of superconducting coils are arranged on both sides of a truck.
As shown in FIG. 2, persistent current switches 2a.about.2f and protective resistors 3a.about.3f are connected to superconducting coils 1a.about.1f, respectively. Heretofore the protective resistors 3a.about.3f have been selected to have a resistance value R that will not induce a normal conductive transition in the superconducting coils when the persistent current switches 2a.about.2f are changed over to the open state.
More specifically, on the basis of actual results obtained with superconducting coils for use in a magnetically levitated train, the resistance value R selected is such that a time constant .tau.=L/R of current attenuation will be approximately 40 sec with regard to a self-inductance L (=1.5.about.4 H). For example, in the case of the superconducting magnets used in the above-mentioned magnetically levitated train MLU 002, the resistance value of the protective resistors is set at 0.07.OMEGA. when the self-inductance of the superconducting coils is 3 H.
Further, in FIG. 2, numeral 4 denotes a demagnetizer for changing over the persistent switches 2a.about.2f to the open state, and numeral 5 denotes a power supply used when energizing and de-energizing the superconducting coils 1a.about.1f. When the superconducting coils 1a.about.1f are in a persistent-current mode (the superconducting state), i.e., when the vehicle is running, the superconducting coils 1a.about.1f are cut off from the power supply 5 by disconnectors 6. Numerals 7a.about.7g designate current lead wires, 8a, 8b power-supply cables, 10a, 10b superconducting magnets and 12 the truck.
FIG. 3 is a diagram illustrating the positional relationship between superconducting coils and ground coils for guidance, as well as the magnetic flux, which prevails when a magnetically levitated train is running in the vicinity of an equilibrium position owing to a magnetic guiding force. Numerals 13a, 13b denote ground coils for guidance, and numeral 15 represents a null-flux line interconnecting the guiding ground coils 13a and 13b.
In a case where the superconducting magnet apparatus is not operating abnormally and the train is being levitated in the vicinity of the equilibrium position by a magnetic guiding force, the magnetic fluxes produced by the superconducting coils 1a, 1b interlink with fluxes of substantially the same magnitude with respect to the guiding ground coils 13a, 13b, respectively, on both sides, as illustrated in FIG. 3.
The guiding ground coils 13a, 13b on both sides are null-flux connected by the null-flux line 15. Consequently, when fluxes of approximately the same magnitude interlink, the induced voltages of the guiding ground coils 13a and 13b cancel each other out and the current becomes small. Accordingly, the electromagnetic forces between the superconducting coils 1a, 1b and guiding ground coils 13a, 13b also become small and so does the magnetic guiding force.
FIG. 4 is a diagram illustrating the positional relationship between the superconducting coils and the ground coils for guidance, as well as the magnetic flux, which prevails when the train is running while undergoing a displacement in the horizontal direction from the equilibrium position.
In a case where the superconducting magnet apparatus is not operating abnormally and the train has been displaced in the horizontal direction from the equilibrium position, the spacing between the superconducting coil 1b and guiding ground coil 13b on the left side differs from the spacing between the superconducting coil 1a and guiding ground coil 13a on the right side, as shown in FIG. 4, and therefore a difference is produced in the magnetic fluxes interlinking with the respective guiding ground coils.
Since the left and right ground coils for guidance are null-flux connected by the null-flux wire 15, a repulsive force acts between the guiding ground coil 13a having the larger linkage flux and the superconducting coil 1a, and an attractive force acts between the guiding ground coil 13b having the smaller linkage flux and the superconducting coil 1b. As a result, a restoring force, namely a magnetic guiding force, that attempts to restore the displacement to the equilibrium position is produced.
If a superconducting coil undergoes a normal conductive transition from the superconductive state for some reason while the train is running, the current of this superconducting coil will be attenuated to zero in several seconds and the magnetic flux interlinked with the guiding ground coil will also decrease sharply.
FIG. 5 is a diagram illustrating the positional relationship between the superconducting coils and the ground coils for guidance, as well as the magnetic flux, which prevails when the superconducting coil on one side of a train undergoes a normal conductive transition.
When the superconducting coil 1a has undergone a normal conductive transition, the magnetic flux interlinking with the guiding ground coil 13a becomes almost zero, as shown in FIG. 5. On the other hand, since the magnetic flux interlinking with opposing guiding ground coil 13b remains almost unchanged, a very large difference is produced between the magnetic fluxes interlinked with the left and right guiding ground coils irrespective of the traveling position of the vehicle, and a large current is induced in the guiding ground coils null-flux connected by the null-flux line 15.
As a result, a large electromagnetic repulsive force is produced between the guiding ground coil 13b and the superconducting coil 1b that has not undergone the normal conductive transition, and this causes the train to be pushed against the guideway side wall on the side of the superconducting coil 1a that has experienced the normal conductive transition.
In order to deal with this problem, the specification of Japanese Patent Application Laid-Open No. 58-148601 discloses a method of serially connecting the superconducting coils electrically every pair of left and right coils. With this method, the currents in the left and right superconducting coils are rendered equal at all times even if the superconducting coil on one side undergoes a normal conductive transition. Consequently, the symmetry of the magnetic fields is maintained and an abnormal electromagnetic force is not produced in the lateral direction.
However, a difficulty is encountered in that since it is required for the left and right superconducting coils to be connected by superconducting lines, vacuum-insulated piping for cooling the superconducting lines must be disposed between the left and right superconducting coils. Since the left and right superconducting coils of the traveling magnetically levitated train possess independent modes of vibration, the vacuum-insulated piping undergoes deformation such as twisting and bending and cracks readily develop in the vacuum-tight portions. The proposed method has not yet been put into practical use for this reason.
Further, the specification of Japanese Patent Application Laid-Open No. 58-140103 discloses a proposal in which, when a superconducting coil undergoes a normal conductive transition, a demagnetizer (or a superconductive breakdown sensor) feeds a current into a heater attached to the superconducting coil so that the opposing superconducting coil is made to undergo a normal conductive transition. With this method there is no need for cooling piping connecting the left and right superconducting coils and, hence, there is no influence from vibration.
However, sufficient reliability is not assured with regard to preventing normal conductive transition of a superconducting coil caused by malfunction of the demagnetizer resulting from lightning during traveling of the train, electrical discharge due to frictional electrification or alternating magnetic fields from the ground coils on the sides of the guideway, or normal conductive transition of a superconducting coil as a form of malfunction caused by the heater of the superconducting coil being heated by an induced voltage impressed upon the circuit extending from the demagnetizer to the heater. Since the normal conductive transition of an energized superconducting coil is accompanied by the evaporation of valuable liquid helium owing to the heat produced, this must be avoided to the utmost except in case of emergencies. Thus, since the method of attaching a heater to the superconducting coil does not assure reliability in terms of preventing malfunction, this method has not been put into practical use so far.
In the conventional superconducting magnet apparatus for magnetically levitated trains of the kind shown in FIG. 2, the superconducting coil 1b produces an induced voltage when it undergoes a normal conductive transition or when the persistent current switch 2b is changed over to the open state. Accordingly, the practice heretofore has been to sense this induced voltage by the demagnetizer 4 and cause the demagnetizer 4 to change over both of the opposing persistent current switches 2a, 2b to the open state.
If the persistent current switches 2a, 2b are changed over to the open state for some reason in this conventional superconducting magnet apparatus, the currents of the superconducting coils flowing into the persistent current switches 2a, 2b are commutated to protective resistors 3a, 3b of small resistance value so that the currents will be attenuated and de-energization achieved without the superconducting coils undergoing the normal conductive transition.
Accordingly, the normal conductive transition of superconducting coils will not be induced even if the demagnetizer 4 malfunctions.
However, when one of the superconducting coils, say the superconducting coil 1b, experiences a normal conductive transition for some reason, the opposing persistent current switch 2a is merely changed over to the open state and the opposing superconducting coil 1a does not undergo a normal conductive transition since the resistance value of the protective resistor 3a is a low 0.07.OMEGA. and, hence, the current is attenuated slowly. On the other hand, the superconducting coil 1b that has undergone the normal conductive transition is de-energized in several seconds whereas it takes several minutes for the opposing superconducting coil 1a to de-energize. Consequently, the magnetic field symmetry on the left and right sides is lost completely and there is almost no effect in terms of mitigating the abnormal electromagnetic force that thrusts the train against the side wall.
Accordingly, a large amount of structural material for strengthening the guideway is required and the cost of constructing the guideway in inevitably very high. This is a particularly serious problem considering the long distance of the line between such cities as Tokyo and Osaka.
Thus, when the superconducting coil on just one side of a magnetically levitated train undergoes a normal conductive transition, the train is thrust against a side wall of the guideway by an excessively large electromagnetic force. In order to prevent this, it is required that the superconducting coil on the opposite side opposing the superconducting coil that has undergone the normal conductive transition also be made to undergo the normal conductive transition so that the symmetry of the magnetic fields may be maintained. In addition, the mechanism that causes the opposing superconducting coil to undergo the normal conductive transition must be easy to realize, highly reliable and fail-safe.